The present disclosure relates generally to aircraft control systems and, in particular, to a nacelle control system in a tilt rotor aircraft.
Tilt rotor aircraft are hybrids between traditional helicopters and traditional propeller-driven aircraft. Typical tilt rotor aircraft have rotor systems that are capable of articulating relative to the aircraft fuselage. The signature characteristic of the tilt rotor is a moveable nacelle which allows the pilot to vector thrust throughout the full range of operating speeds. Tilt rotor aircraft are capable of converting from a helicopter mode, in which the aircraft can take-off, hover, and land like a helicopter; to an airplane mode, in which the aircraft can fly forward like a fixed-wing airplane. The desired flight path is achieved through proper coordination of a thrust control lever, a longitudinal stick, a lateral stick, and nacelle position controls.
In the V-22 Osprey aircraft, tilt rotor technology combines vertical takeoff and landing performance of a helicopter with the speed and range of a turboprop airplane. The aircraft flight controls comprise a center stick that is used to control pitch and roll, foot pedals to control yaw, a thrust control lever to control thrust, and a thumbwheel to control nacelle position. The first three flight controls are common to conventional aircraft (with the thrust control lever representing a combination of rotorcraft collective and airplane throttle), but the thumbwheel that controls nacelle position is common only to tilt rotor aircraft. The thumbwheel provides control of the nacelles over a range of 0 to 97.5 degrees. The thumbwheel commands three aircraft configurations: airplane, helicopter, and an airplane/helicopter (hybrid) mode. Airplane mode is defined when nacelles are placed at 0 degrees, and helicopter mode is defined when nacelles are placed at greater than 80 degrees. Nacelle settings between 0 and 80 degrees are defined as conversion mode, which is a hybrid of the airplane and helicopter modes. During conversion to airplane mode, the cyclic and collective rotor controls phase out to permit control by conventional aircraft flaperons, rudders, and elevator. The rate at which the nacelles move is proportional to the pilot's thumbwheel displacement. Nacelle modulations may range from 0 deg/sec to a maximum rate of 8.0 deg/sec. Upper conversion corridor protection and lower conversion protection, which limit the nacelles' range of movement as a function of airspeed and altitude, are provided to reduce structural damage and the tendency to stall the aircraft in flight.
The main difference between a conventional turboprop aircraft and the V-22 aircraft is that the inputs from both the longitudinal and vertical axes control speed, altitude, or a mixed variation of speed and altitude depending upon the nacelle setting, as summarized below.
AXIS VERSUS NACELLE SETTNGSAxis InputNacelle AngleOutputLongitudinal0degAltitudeLongitudinal90degSpeedLongitudinal45degSpeed/AltitudeVertical0degSpeedVertical90degAltitudeVertical45degSpeed/Altitude
Pilots must modulate the angle of the nacelles within a conversion corridor while making inputs to the longitudinal and vertical axes to achieve a desired speed and altitude.
Flight directors perform the basic task of translating flight guidance commands into a logical set of display symbology. The pilot responds to the display symbology and applies control system inputs to guide the aircraft along a desired flight path. During operation of a tilt rotor aircraft, nacelle control is required with all flight director speed modes, either coupled or uncoupled. Uncoupled operations require the pilot to follow flight director cues by manipulating the aircraft controls appropriately. During coupled operations, the flight control system automatically minimizes flight director guidance errors. The following flight director speed modes require nacelle control: Airspeed Select/Hold (SPD SELECT), Approach to Hover (AHOV), and Depart from Hover (DHOV/GA)
Aside from making inputs to each control axis, pilots are faced with other cockpit management tasks. Pilots are continuously scanning the operating conditions of the aircraft and monitoring system status. The V-22 cockpit contains multiple instrument displays that must be monitored on a regular basis during flight operations. Providing a nacelle position indicator that suggests to the pilot the optimum nacelle position for a given desired speed reduces pilot workload and increases safety of flight. Eliminating altogether the need to modulate nacelle angle manually (fifth axis) by providing an automated nacelle conversion function decreases workload further, improves safety of flight, improves handling qualities, and increases mission success rate.
The automatic nacelle conversion capability for tilt rotor aircraft can be improved through the use of additional guidance and atmospheric data.